The invention relates to a receiver and a reception method in a data transmission system wherein a signal to be transmitted can comprise information transmitted in a frame form by using several different transport formats and in which method the transport formats used at a given time are indicated in the frame. The invention relates Particularly to how errors can be detected in indicating a transmission rate.
Requirements set to data transmission systems are on the increase. This relates particularly to wireless data transmission systems, such as cellular radio systems that are to provide more and more accomplished services, such as different data and video services, for example.
Conventionally, wireless data transmission systems have only been used for transmitting speech. The increasing number of different services to be transmitted particularly in wireless systems means that a system must be capable of transmitting over a radio path signals with different capacities, such as speech at a data rate of 8 kbit/s and data at a rate of 64 kbit/s. In addition, it is also necessary to transmit signals that have different quality levels, the signals typically requiring also different data rates simultaneously. A typical example is a video connection wherein image necessitates a high data rate and quality level while voice allows to be transmitted at a lower data rate and quality level. Hence, the data transmission system should be capable of operating efficiently in an environment where transmissions of several different data rates, quality levels and service types are transmitted.
It is typical of digital data transmission systems in particular that signals are transmitted in a frame form. A frame comprises a plurality of time slots. In systems where data can be transmitted by several different transport formats it is necessary to indicate in each frame which transport format is used in the frame. The different transport formats are often implemented by variations either in the size of the data part of a time slot or in the coding used in connection with transmitting the data. In principle, the transport format can vary frame by frame or time slot by time slot.
Since the quality of signals varies, it is obvious that different coding is used for signals of different types. Hence, the depth of channel coding, for example, varies according to different signals. This can be exemplified by speech and video connections, on which a coding depth of the length of one frame is typically used for speech, and a channel coding depth of the length of k frames, where k greater than 1, is typically used for video. The transport format and coding depth used at a given time are indicated by transport format indicators TFI that are transmitted over a dedicated control channel.
When one or more services with the same channel coding depth is/are transmitted over one radio link, coding of the transport format indicators can be implemented over the same time period as data, as is the case in the prior art solutions. The coding of the transport format indicators must then be carried out according to a service using the smallest coding depth in order to avoid additional delays on the radio link. The problem with the prior art solution is that the transport format indicators cannot utilise the same interleaving as the data that has a greater coding depth. Thanks to interleaving, the bit error ratio BER of the data decreases; this does not, however, apply to the transport format indicators.
For the above reason, transport format indicators may be more susceptible to errors than data bits. If the transport format indicators are erroneous, the demultiplexing of services and the setting of transmission rates are carried out erroneously at the receiver. The input of a viterbi- and turbo-decoder receives random bits, which means that the contents of a frame processed erroneously cannot be decoded in a correct manner.
An object of the invention is thus to provide a method and an apparatus implementing the method such that the problems mentioned above can be solved. This is achieved by a reception method in a data transmission system of the invention, in which system a signal to be transmitted can comprise information transmitted in a frame form by using several different transport formats, in which frames also information on the transport format used is transmitted by means of transport format indicators, and in which system the information to be transmitted is coded by using a coding depth depending on the properties of the information to be transmitted, at least some of the coding depths being multiples of a given basic coding depth TK. In the method of the invention, when a signal is received which is known to contain information coded by using a coding depth of n*TK, if it is detected that nxe2x88x92x frames have been received, where a positive integer x is lower than n, in which frames transport format indicators have been identified indicating that the frame comprises the information coded using the known coding depth n*TK, and x frames wherein the transport format indicators do not indicate that the signal comprises the information coded using the known coding depth n*TK, it is inferred that the x frames in question have been received erroneous.
The invention further relates to a receiver in a data transmission system wherein a signal to be transmitted can comprise information that is transmitted in a frame form by using several different transport formats, in which frames also information on the transport format used is transmitted by means of transport format indicators, and in which system the information to be transmitted is coded by a coding depth depending on the properties of the information to be transmitted, at least some of the coding depths being multipies of a given basic coding depth TK When a signal which is known to comprise information coded by using a coding depth of n*TK is received at a receiver of the invention, the receiver of the invention comprises means for detecting that nxe2x88x92x frames have been received, where a positive integer x is lower than n, and means for identifying from the frames the transport format indicators indicating that the frame comprises the information coded using the known coding depth n*TK, and means for detecting that x frames have been received in which the transport format indicators do not indicate that the signal comprises the information coded using the known coding depth n*TK, and means for inferring that the x frames in question have been received erroneous.
The preferred embodiments of the invention are disclosed in the dependent claims.
The invention is based on the idea that when it is known that a service whose coding depth is n frames is about to be received, and that frames which should, according to the number of previous frames, include said service are about to be received, and when such transport format indicators are detected according to which the frame does not include said service, it can be inferred from this that an error has occurred in the reception.
Several advantages are achieved by the method and arrangement of the invention. Since errors can be detected in reception already prior to decoding, decoding errors can be decreased by setting the erroneous bits to zero since bits set to zero cause a smaller error than random erroneous bits do.